Method of producing a coated ferrous substrate

ABSTRACT

The invention concerns a method of producing a coated ferrous substrate having improved spot welding characteristics comprising introducing a ferrous substrate into a bath of molten metallic coatant so as to coat the substrate therewith, withdrawing the coated substrate from the bath, and thereafter directing particles of metallic iron or iron-based alloy onto the coated substrate, before the molten coatant thereon solidifies, so that the particles are retained adjacent the surface of the coatant during solidification of the latter.

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1 51 Jan. 2, 19731 {75] Inventors: Albert Edward JacksonfGvvernaF field Mold; Richard Fred Williarn's, Deeside, both of England [73] Assignee: British Steel Corp, llondonf England [22] Filed: 0712122, 1970 H {21} Appl. No.: 77,505

[30] Foreign Application Priority Data Oct. 9, 1969 Great Britain ..49,614/69 [52} US, (Cl. ..1117/311, 29/1961, 29/1962,

[51] int. Cl ..lflddd 11/094, 344d 1/34 [58] Field 05 Search 17/16, 26, 29, 31, 33, 71 M, 117/131, 204,207

[56] Reierences (Iited UNITED STATES PATENTS 2,714,563 8/1955 Poorman et a1. ..117/16 2,964,419 12 1960 Link et al. ..117/16 2,354,113 7 1944 0661a ..117/31 3,017,689 l/1962 Link 61 al. ..117/26 3,155,530 11/1964 Schnedler.... ..117 26 3,586,614 6/1971 Boggsetal ..117/71 M Primary Examiner-William D. Martin Assistant Examiner-Sadie L. Childs Att0rneyMawhinney & Mawhinney 57 ABSTRACT The invention concerns a method of producing a coated ferrous substrate having improved spot welding characteristics comprising introducing a ferrous substrate into a bath of molten metallic coatant so as to coat the substrate therewith, withdrawing the coated substrate from the bath, and thereafter directing particles of metallic iron or iron-based alloy onto the coated substrate, before the molten coatant thereon solidifies, so that the particles are retained adjacent the surface of the coatant during solidification of the latter.

11 Claims, 2 Drawing Figures PATENTEDJAH 2 1915 SHEET 2 BF 2 N wI METHUD 6F PRODUCING A COATED FERROUS SUESTRATE This invention concerns a method of producing a coated ferrous substrate having improved spot welding characteristics, and, although the invention is not so restricted, it is more particularly concerned with hot dip galvanizing.

Although the invention is primarily directed to any novel integer, step, or product, or combination of such integers or steps, as herein disclosed, nevertheless according to one particular aspect of the present invention, to which however the invention is in no way restricted, there is provided a method of producing a coated ferrous substrate having improved spot welding characteristics comprising introducing a ferrous substrate into a bath of molten metallic coatant so as to coat the substrate therewith, withdrawing the coated substrate from the bath, and thereafter directing particles of metallic iron or iron-based alloy onto the coated substrate, before the molten coatant thereon solidifies,

so that the particles'are retained at or near the surface of the coatant during solidification of the latter.

The term metallic iron is to be understood as exeluding iron compounds.

We have found that the use of particles of metallic iron or iron-based alloy improves the spot welding characteristics of the product without impairing the corrosion resistance thereof.

For example, we have found that one particular hot dip galvanized steel produced by the method of the present invention, metallic iron powder being used, had an electrode life of about 15,000 spot welds,.by comparison with an electrode life of 1,500 spot welds in the case of a similar hot dip galvanised steel produced by a conventional process. Moreover, an even higher electrode life is obtainable if, after the molten coatant has solidified, an outermost layer of metallic iron is deposited onto, without being diffused into, the coatent.

The conventional process of hot dip galvanizing, moreover, produces a product having a spangled surface which may be visible even when coated, For example, if a relatively thin stove enamel is applied as a decorative finish to a galvanised steel product (e.g., a refrigerator, washing machine, shelving, or a motor car), the existence of the Spangled surface is undesirable since its contours can be seen in the enamel. The method of the present invention may, however, be so operated as to overcome this problem.

. Preferably, the.saicl particles are directed onto the coated substrate so as to reach the latter at a velocity not exceeding 500 feet per minute, since at higher speeds the particles may bounce off the coated substrate, thus reducing weldability.

Preferably, the quantity of particles directed onto each square foot of the surface of the coated substrate is in the range of l to 3 grams.

Moreover, a second supply of the said particles may be directed onto the coated substrate after the application thereto of the first supply thereof but before the molten coatant solidifies, the said second supply being smaller in quantity than the first supply and being directed onto the coated substrate so as to reach the latter at a velocity greater than that of the first supply. This is desirable in cases where the substrate is thick (e.g., greater than 0.048 inch in thickness) to ensure that the coatant does not remain molten after the application of the said particles, since otherwise some spangles may form. It is believed that the powdered metallic iron or iron alloy creates nuclei on which the zinc or other coatant will solidify.

The said particles may be formed from pure iron, a mild steel, or an iron based alloy such as one having percent iron and 20 percent zinc.

The substrate is preferably a metallic strip which is continuously moved through the bath. The excess molten coatant on the metallic strip may be removed by passing the latter through the nip of a pair of rolls. Alternatively, the excess molten coatant may be blown back, by a controlled blast of air, into the bath.

The strip may be of a low alloy steel e g., that sold under the Trade Mark CORTEN which contains a small amount of copper and chromium.

The molten coatant is preferably zinc and in this case the particles are preferably directed onto the molten zinc when the temperature of the latter is in the range 440 to 450 C.

However, the method of the present invention is also applicable to the provision of a mild steel strip with an aluminum or aluminum alloy coatant, e.g., a zinc-aluminum alloy coatant, an aluminum-magnesium alloy coatant, or an aluminum-silicon alloy coatant.

The size of the said particles is not normally of great importance, but it is preferred that they should be not greater than 200 mesh in size.

The equipment employed to direct the said particles onto the coated strip should be movable to take account of the gauge of the strip. Thus if the strip is arranged to move vertically upwardly from the bath, the said equipment should be capable of being moved up or down to take account of the varying position of the solidification zone of the coatant. That is to say, a light gauge strip cools more quickly than a heavier gauge strip, and the solidification zone of the coatant is therefore nearer to the bath in The case of the light gauge strip than it is in the case of the heavier gauge strip. Consequently, the lighter the gauge of the strip being used, the nearer the said equipment needs to be disposed to the bath.

' The invention is illustrated, merely by way of example, in the accompanying drawings, in which:

FIG. 1 is a schematic elevation of a hot dip galvanizing plant for use in the method of the present invention, and

FIG. 2 is a section through a galvanized steel strip produced by the method of the present invention.

In FIG. 1 there is shown a hot dip galvanizing plant having an entry zone 10, a surface preparation zone it, a coating zone 12, and a delivery zone 13. Mounted in the entry zone 10 is a de-coiler 14 from which a steel strip 15 may be de-coiled. The strip 15 is moved con tinuously through the surface preparation zone 1 1 so as to pass successively therein through an apparatus 16 in which the strip 15 is subjected to a grease burn at about 400 C, and then through an apparatus 17 in which the strip is heated in a reducing atmosphere at about 700 C.

The strip 15 then passes over a roller 20 and then vertically downwards so as to pass around a roller 21 in a bath 22 which is disposed in the coating zone 12, the bath 22 containing molten zinc.

The strip 15, after having been thus coated with a layer 23 (FIG. 2) of molten zinc, as a result of its introduction into the bath 22, then passes upwardly through the nip of a pair of rolls 24 by means of which excess molten zinc is removed, after which it passes through powder applicator equipment 25.

The powder applicator equipment 25 comprises a pair of bottom applicators 26 and a pair of top applicators 27, each pair of applicators consisting of two powder applicators arranged on vertically opposite sides of the strip 15.

The bottom and top applicators 26, 27 are respectively provided with supply conduits 30, 31 through which metallic iron or iron-based alloy particles 32, (see FIG. 2) whose melting point is above 450 C and whose size is preferably not greater than 200 mesh, may be carried in a stream of air (or other gas) towards the. zinc coating on the strip 15. The particles'32 supplied by the bottom applicators 26 reach the coated strip at a velocity in the range 300 to 500 feet per minute, while the quantity of the particles 32 directed from the bottom applicators 26 into each square foot of each surface of the strip is in the range 1 to 3 grams. The particles 32 are directed onto the layer 23 of molten zinc before the latter solidifies so that the particles are retained at or near the surface 33 of the layer 23 of zinc coatant during solidification of the latter. As indicated above, the use of the particles 32 from the bottom applicators 26 very substantially improves the electrode life of the product and, moreover, except in the case of heavy gauge steel strip, i.e., of a thickness exceeding 0.048 inch, gives the strip a matte, spanglefree surface.

If the strip 15 is of heavy gauge, however, the top applicators 27 are provided with a second supply of particles 32 which reach the molten zinc on the strip at a velocity in the range of 500 to 1000 feet per minute, the amount of the second supply which is directed onto each surface of the zinc coatant prior to solidification of the latter being in the range of 0.1 to 0.2 grams per square foot.

The low velocity of the air from the bottom applicators 26 is insufficient, in the case ofa heavy gaugestrip having a high heat content, to cool the latter below the zinc recrystallization temperature and therefore insufficient to suppress spangles completely. However, in this case, the application of the small quantity of particles 32 from the top applicators 27, which produce a high air velocity, will suppress the spangles and produce a matte surface.

The powder applicator equipment 25 is, as indicated by arrows 34, movable longitudinally of the strip 15 towards and away from the bath 22 so that it may be positioned below the solidification zone of the zinc, and preferably in the region where the temperature of the latter is in the range 440 to 450 C. Excess powder is withdrawn from the powder applicator equipment 25 by a pump 35, whereby the excess powder is recovered.

The strip 15, after leaving the powder applicator equipment 25, passes over a top roller 36 and is then coiled on a coiler 37 in the delivery zone 13, the layer 23 of zinc having solidified prior to the strip 15 reaching the top roller 36.

After removal from the coiler 37, the strip 15 may have its weldability still further improved by providing it with an outermost layer 40 (FIG. 2) of metallic iron.

The layer 40 may be applied electrolytically but is preferably an immersion deposit of iron which fills in' the gap between the particles 32. This immersion deposit may, for example, be produced by dipping the strip, for 5 seconds, in a solution of ferrous chloride which is maintained at 40 C, has a pH of 2.2, and contains 56 grams/litre of iron, the strip being thereafter washed and dried. It was found that the layer 40 raised the electrode life of the strip from 15,000 to 17,000 spot welds, and that this layer 40, in spite of its being a layer of metallic iron, had excellent corrosion resistance.

We claim:

1. A method of producing a coated ferrous substrate having improved spot welding characteristics comprising introducing a ferrous substrate into a bath of molten zinc, aluminum, or aluminum alloy coatant so as to coat the substrate therewith, withdrawing the coated substrate from the bath, and thereafter directing particles of metallic iron or iron-based alloy having a mesh size not'exceeding 200 mesh onto the coated substrate,

before the molten coatant thereon solidifies, the said particles being directed onto the coated substrate so as to reach the latter at a velocity not exceeding 500 feet per minute, so that the particles are retained adjacent the surface of the coatant during solidification of the latter.

2. A method as claimed in claim 1 in which the said velocity is in the range of 300 to 500 feet per minute.

3. A method as claimed in claim 1 in which the quantity of particles directed onto each square foot of the surface of the coated substrate is in the range of l to 3 grams.

4. A method as claimed in claim 1 in which a second supply of the said particles is directed onto the coated substrate after the application thereto of the first supply thereof but before the molten coatant solidifies, the said second supply being smaller in quantity than the first supply and being directed onto the coated substrate so as to reach the latter at a velocity greater'than that'of the first supply.

5. A method as claimed in claim 4 in which the velocity of the second supply is in the range 500 to 1000 feet per minute.

6. A method as claimed in claim 4 in which the amount of the said second supply which is directed onto each surface of the coated substrate is in the range of 0.1 to 0.2 gram per square foot.

7. A method claimed in claim 1 in which, after the molten coatant has solidified, an outermost layer of metallic iron is deposited onto, without being diffused into, the coatant.

8. A method as claimed in claim 1 in which the particles are directed onto the coated substrate by being carried in a gas stream. I

9. A method as claimed in claim 1 in which the substrate is a strip which is continuously moved through the bath.

10. A method as claimed in claim 9 in which the excess molten coatant on the strip is removed by passing the latter through the nip of a pair of rolls.

U. A method as claimed in claim 9 in which the excess molten coatant on the strip is blown back into the bath. 

2. A method as claimed in claim 1 in which the said velocity is in the range of 300 to 500 feet per minute.
 3. A method as claimed in claim 1 in which the quantity of particles directed onto each square foot of the surface of the coated substrate is in the range of 1 to 3 grams.
 4. A method as claimed in claim 1 in which a second supply of the said particles is directed onto the coated substrate after the application thereto of the first supply thereof but before the molten coatant solidifies, the said second supply being smaller in quantity than the first supply and being directed onto the coated substrate so as to reach the latter at a velocity greater than that of the first supply.
 5. A method as claimed in claim 4 in which the velocity of the second supply is in the range 500 to 1000 feet per minute.
 6. A method as claimed in claim 4 in which the amount of the said second supply which is directed onto each surface of the coated substrate is in the range of 0.1 to 0.2 gram per square foot.
 7. A method claimed in claim 1 in which, after the molten coatant has solidified, an outermost layer of metallic iron is deposited onto, without being diffused into, the coatant.
 8. A method as claimed in claim 1 in which the particles are directed onto the coated substrate by being carried in a gas stream.
 9. A method as claimed in claim 1 in which the substrate is a strip which is continuously moved through the bath.
 10. A method as claimed in claim 9 in which the excess molten coatant on the strip is removed by passing the latter through the nip of a pair of rolls.
 11. A method as claimed in claim 9 in which the excess molten coatant on the strip is blown back into the bath. 